You might notice that, aside from the "A" and "B" values, the IVs generated for bred Pokémon match those generated for method 1 Pokémon. This is because the game uses method 1 to generate the Pokémon's IVs before it replaces them with the pre-set values from the parents. However, there are odd occasions when the game opts to either use a different method from method 1 to generate IVs, or assign "A" and "B" values in a different way. What exactly triggers this event is unknown, but most of the results have been documented.
While searching for spreads, you will notice that certain spreads have an "S" after their frame. This means the frame in question is what is known as a "split" spread. Split spreads earn their name because they get the first half of their IVs from the "normal" spread that occurs on that frame, and the second half of their IVs from the spread that follows (the IVs they inherit are taken from the spread that follows). This is due to being created by method 4, rather than method 1. While split spreads occur at the same time as their normal counterparts (for example, 943S occurs at the same time as 943). Split spreads are the most common non-traditional spreads that you will encounter, though there is no known way to hit them consistently, other than through trial and error.
Another less common anomaly is the game going into what is known as "Alternate Inheritance" mode. Alternate Inheritance mode causes the IVs inherited by the parents to shift backwards one frame. This means that the IVs passed from the parents on frame 943 when using the vanilla method 1 generation will be inherited on frame 942 in Alternate Inheritance mode. This completely changes what spreads are viable; better spreads may occur with shorter wait times, and different parents than usual. However, Alternate Inheritance mode is very unorthodox in its appearances; when it occurs, it will be the method of IV generation for an indefinite amount of time, liable to disappear without notice. For this reason, it is best not to aim for an Alternate Inheritance spread right away; wait until you see different IVs being inherited to take advantage of it.